Mercury vapor is used as a fluorescent light source and in its natural state contains about 0.15% of the isotope .sup.196 Hg. It would be desirable to enrich the .sup.196 Hg content of mercury vapor, since it has been found that enrichment to a level of about 3% .sup.196 Hg increases the efficiency of a conventional fluorescent lamp by approximately 5%. Enrichment of an atomic vapor in one or more isotopes may be achieved through separation processes which operate on very small differences between the chemical or physical properties of the isotopes. One method of isotope enrichment, utilized in enriching .sup.235 U, operates by selective excitation of energy states of one uranium isotope without corresponding excitation and ionization of the other uranium isotopes. In achieving a high degree of ionization of the selected isotope, the radiation, which is typically laser radiation, must be chosen to have certain specific photon energies which correspond to particular radiative transitions in the selected isotope .sup.235 U, but not of the other isotopes.
However, selection of the energies to be used for excitation, as well as the sources of the radiation energy which is used, is not readily apparent, particularly when working in a system other than uranium vapor. Therefore, with regard to mercury vapor it would be desirable to develop a system whereby precise energies for the selective excitation and photoionization of .sup.196 Hg, as well as radiation sources for these energies, can be determined and utilized.